This application is based on Japanese Patent Application No. 2001-216589 filed in Japan, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a sintered body, a method for its surface densitication, a process of working and manufacturing an electrode by this method, and a circuit breaker such as a vacuum vessel.
2. Description of the Related Art
Vacuum circuit breakers are devices which open and close high voltage and large electric current by opening and closing the path between a movable electrode and a fixed electrode which are placed in a vacuum container. In such a vacuum circuit breaker, an electric arc is formed between the movable electrode and the fixed electrode at the time of circuit breaking. This arc is considered to be an ionized gas or hot electrons of the material component of electrodes. The arc between the movable electrode and the fixed electrode disappears once this ionized gas has sufficiently diffused. However, when reignition (restriking) voltage rises before that, it causes the arc to be again formed between the movable electrode and the fixed electrode to make circuit break impossible. Accordingly, in order to avoid such a phenomenon, vacuum circuit breakers are required to have a high circuit-break performance.
This breaking performance of vacuum circuit breakers is known to be greatly influenced by the material properties of arc electrode portions at the part facing the arc running faces of electrodes, and experiments are made using a variety of material systems. As the result, as materials for the electrodes, it has been considered preferable to use melted-and-forged alloys such as Cuxe2x80x94Bi, Cuxe2x80x94Te or sintered alloys such as Cuxe2x80x94Mo, Cuxe2x80x94W.
The electrodes of the vacuum circuit breakers are also required to have performances such that they can handle a large circuit-break current, have a high breakdown strength, have a sufficient conductivity to cause less heat generation, and do not cause any fusion bond between the movable electrode and the fixed electrode. Accordingly, a Cuxe2x80x94Cr alloy is in wide used, as satisfying all the performances in a relatively well balanced state. In this material system, also used are materials to which the third element such as Al, Si, Ta, Nb, Be, Hf, Ir, Pt, Zr, Si, Rh or Ru has been added.
As a method of manufacturing such electrodes of vacuum circuit breakers, a method making use of sintering is inexpensive, and has recently become widely used. When, however, the electrodes are manufactured by a sintering process, there has been a problem that 1 to 10 percentage of voids may remain in the interiors of the electrodes even after the sintering thereby to make the electrodes have a low conductivity.
Electrodes having a high porosity have so low a conductivity as to have a low thermal diffusivity and besides to generate more Joule heat, so that the temperature may greatly rise when the electrodes are electrified.
Hence, the arc running faces of the electrodes tend to deteriorate. Also, referring to the circuit-break performance of vacuum circuit breakers, the temperature rise occurs at arc electrodes. Hence, more metallic elements vaporize and ionize at the time of circuit break of the vacuum circuit breaker thereby to cause a delay in attenuation of the arc and a lowering of breaking performance of the vacuum circuit breaker.
Hence, it is preferable for the electrodes to have a high density. Accordingly, in the case when the electrodes are prepared by sintering, various methods are employed in order for the electrodes to be improved in density.
For example, as a method commonly used to improve the relative density of materials after sintering, sinter forging is available in which the materials are forged after sintering as they are kept at a high temperature. This conventional sinter-forging, however, is very expensive for both forging equipment and forging molds and requires great equipment investment.
As a method of improving the density only at the surface, shot peening disclosed in Japanese Patent Application Laid-open No. 49-17311 is known in the art. However, this shot peening, too, requires equipment exclusively used therefor, resulting in great equipment investment, and besides it has a disadvantage that a workpiece to be worked may chip when it is brittle.
A method in which a sintered product is compressed by rolling after sintering is also disclosed in Japanese Patent Application Laid-open No. 8-143910. However, this method making use of surface rolling, too, requires great equipment investment like the above methods. Also, working objects are inevitably limited to plate-like products.
As disclosed in Japanese Patent Application Laid-open No. 11-250783, it is further attempted to use a Cuxe2x80x94TiC alloy in arc electrode materials for vacuum vessels. Sinter infiltration is also used as a method by which the electrodes are improved in density while their compositional distribution is kept uniform, and electrodes made integrally of materials having different physical properties have been put into practical use in order to make the electrodes have higher function. For example, Japanese Patent Application Laid-open No. 7-29461 discloses an integrally infiltrated electrode in which a arc electrode material which is usually an alloy of metals of two or more types and its electrode support member which is a single-phase alloy of a high-conductivity material such as Cu are made into an integral structure which is metallographically continuous structure so that the mechanical strength can be improved and the number of assemblage steps can be reduced.
Recently, as a working method by which electrode performance can be improved, it is proposed as disclosed in Japanese Patent Application Laid-open No. 11-250782 that a working object which is held and kept rotated is worked by cutting away an end of the working object by means of a cemented carbide lathe cutting tool, followed by a first step in which, rotating the working object regularly, it is worked by cutting by means of a diamond lathe cutting tool, and then a second step in which, rotating the working object reversely, the worked surface of the working object is finished by burnishing using a flank face of a diamond lathe cutting tool set more extended by 0 mm to 0.005 mm than that of the first step, to improve surface roughness of the worked surface of the working object.
This technique is to smooth the worked surface of the working object so that any protrusions coming to be starting points of arc discharge at the time of circuit break can be removed and the circuit-break performance can be improved. This working method can at least meet expectations for the improvement in breakdown strength, but can not improve the conductivity of sintered materials. This is because plate thickness loss necessarily takes place when any porosity kept within a significant range is lessened by working from the surface according to this method in order to improve the performances of electrodes for circuit breakers, but any plate thickness loss can by no means take place beyond the depth of cut. For example, even when the burnishing is performed in a depth of cut of 0 mm to 0.005 mm using the back of a diamond lathe cutting tool for cutting, the plate thickness loss and the interior porosity loss are substantially zero.
An object of the present invention is to provide an inexpensive circuit breaker having superior current break-off performance (breaking performance), an electrode used therein, a manufacturing process and a method for surface densitication which are used for manufacturing the electrode, and a sintered body at least part of the surface of which has been compacted by the method for surface densitication.
To achieve the above object, the present invention provides an electrode comprising an electrode main body formed of the same material as a whole, wherein the conductivity of the electrode main body at its part extending from the arc running face to a stated depth is higher than the conductivity of the whole electrode main body. Here, the above stated depth may be, e.g., a half of the thickness of the electrode main body (i.e., the thickness from the arc running face to the back surface). Incidentally, the back surface is herein meant to be the surface on the side opposite to the arc running face.
In the present invention, the stated depth may also be 2 mm, where the conductivity of the electrode main body at its part extending from the arc running face to a depth of 2 mm can be made at least 1.2 times the conductivity of the whole electrode main body or the conductivity of the electrode main body at its part extending from the back surface thereof to a depth of 2 mm.
The present invention also provides an electrode comprising an electrode main body, wherein the porosity of the electrode main body at its part extending from the arc running face to a stated depth (e.g., 0.5 mm) is lower than the porosity of the whole electrode main body.
In the present invention, the electrode main body may be provided with a through hole extending from the arc running face to reach the back surface, and the arc running face may be provided with a groove. The electrode main body of the electrode of the present invention may preferably comprise a sintered alloy. Also, the electrode main body may preferably have an average porosity of from 1 to 10 vol. %.
The present invention still also provides a circuit breaker comprising the above electrode of the present invention.
The present invention further provides a method for surface densitication, comprising steps of working a working object by cutting away a part of the surface of the working object with a cutting tool to form a worked surface, and working the worked surface by burnishing with a burnishing tool to cause the surface to retreat, to densiticate the worked surface portion by plastic deformation, wherein said working object is held and kept rotated. And the present invention also provides a sintered body having been densiticated at at least part of its surface by such the method for surface densitication.
As the burnishing tool, a milling type burnishing tool may be used. Where the milling type burnishing tool is used, the burnishing can be performed even when the worked surface is previously provided with a groove or grooves and can not be worked by a lathe.
The extent of retreat of the worked surface as a result of burnishing may preferably be 300 xcexcm or less in order to ensure the thickness precision of the electrode. If burnishing conditions are so set as to provide a larger extent of retreat than that, the electrode may have a non-uniform finish thickness because of a non-uniform porosity of its stock product. On the other hand, where an electrode having a porosity of 10% is worked by burnishing under conditions which provide the extent of retreat of 300 xcexcm or less, the porosity in the range of 2 mm from the arc running face, which influences electrode performance, can be made sufficiently small.
For the working object used in this method for surface densitication of the present invention, a sintered body is particularly suited. Preferred are, besides the electrode main body described above, sintered component parts (sintered bodies) which are desired to be made to have a higher strength at particular portions or a higher surface hardness after molding, such as guides, pushes, cam rings, pulleys and gears of automobiles and dynamos.
There are no particular limitations on the worked surface to which the method for surface densitication is applied. It may appropriately be selected according to the shapes of working objects and the purposes of working. For example, outer peripheries, inner peripheries, edgeface and through-hole inner walls of working objects may be set as worked surfaces.
In the case when the worked surface stands provided with a groove, the burnishing may be carried out through a tool path such that the relative movement between the working object and the burnishing tool is in parallel to the worked surface and also the burnishing tool is brought into contact with the whole worked surface. This enables densitication of the worked surface portion except the groove inner wall.
The present invention still further provides a process for manufacturing an electrode, comprising the step of densiticating at least part of the surface of an electrode main body by the above method for surface densitifcation according to the present invention. As steps other than this step of densitication, the process may be provided with, e.g., a molding step of molding a conductor powder as a raw material in the shape of an electrode main body to obtain a molded body, and a sintering step of heating the molded body to effect sintering to obtain the electrode main body.